Pneumonia caused by gram-negative bacilli: an overview.

Colonization of the oropharynx by Pseudomonas aeruginosa and enteric gram-negative bacilli in acutely ill or debilitated patients, alcoholics, diabetics, and persons with chronic bronchitis may lead to pneumonia. Although gram staining of sputum may provide immediate etiologic clues, the diagnosis is proven only by isolation of the pathogen from blood or pleural fluid or by various invasive techniques since expectorated specimens from highly susceptible patients are often contaminated with aerobic gram-negative bacilli colonizing the oropharynx. However, restricting the definition of cases to those involving empyema or bacteremia results in an underestimation of incidence. Combination therapy with a beta-lactam antibiotic plus an aminoglycoside is commonly recommended for gram-negative bacillary pneumonia because (1) the patients involved are usually debilitated and immunocompromised; (2) mortality is high; and (3) the spectrum of antibacterial activity is increased, emergence of resistance may be retarded, and synergistic activity may result. For example, a third-generation cephem antibiotic plus an aminoglycoside can be used for initial treatment of community-acquired gram-negative bacillary pneumonia, and piperacillin or azlocillin plus amikacin can be used for initial treatment of nosocomial infection in which P. aeruginosa or some other antibiotic-resistant gram-negative bacillus is more likely to be involved.